Cavity resonator



April 25, 1961 M. c. THOMPSON, JR, ETAL 2,981,908

CAVITY RESONATOR 2 Sheets-Sheet 1 Filed Dec. 15, 1958 m M w w m mmgm mwd A fifi 65M J Z? %%M CONDUCT/V5 C'OIQT/A/G April 25, 1961 M. c.THOMPSON, JR, ETAL 2,981,908

CAVITY RESONATOR 2 Sheets-Sheet 2 Filed Dec. 15, 1958 INVENTORG M004] 67/20mps0/M Frank EFPee we Dona/0' M Wafers M ATTORNEY Stats z,9 s1,90s

CAVITY RESONATOR Moody C. Thompson, Jr., Boulder, Frank E. Freethey,

Brighton, and Donald M. Waters, Boulder, Colo., assigno'rstothe UnitedStates. of America as represented by the Secretary of Commerce 1 Filedna. 15 1958,.Ser. No.7s0,646

2 Claims. (Cl. 333-83 The present invention relates to cavity resonatorssuch as are employed at radio frequencies in the microwaveregion andparticularly contemplates an improved resonatorwhich has an extremelylow frequency drift with temperature change.

There are numerous occasions in electronic, instrumentation when acavity resonator having a very low,

temperature coefficient is essential. For example, when using a cavityresonator as a frequency'stabilizing element in a klystron oscillator oras the, resonant cavity in connection with amicrowave. refractometer itis important to maintain a fixed frequency of oscillation despitetemperature. changeswhich may afiect the size of'a cavity resonator. V

Presently known procedures to achieve suchresult in volve. thefabricationof the resonant cavity from mate- 'vrials which arerelatively temperatureinsensitive such as Invar in combination withaconductingmedium such' as brass or steel. By properly proportioning therelative dimensions of the cavity-when constructed oflsuch material, itis possible -to obtain compensation to ahigh' degree, for example, aboutone part in million per degrees Centigrade, as an over-allfrequency-temperature coefficient. Howeyer, a variety of dynamic heatingeffects can result from such a composite cavitywhich may ing them willbe further detailed as the description proceeds. r h It isaccordingly animmediate" object of the present inventionto provide an improved cavityresonator which tion. r

Another object ofthisinvention is to provide an improved. cavityresonator which is particularly adaptable The-specific types of2,981,908 Patented Apr. 25, 196 1 '2 resonator assembly embodying theprinciples of themesent invention;

Fig. 2 is a view of the body portion of the cavity resonator of Fig. 1;

Fig. 3 is a view of the ceramic moulding employed in making a cavityresonator;

Fig. 4 is a detailed sectional view showing the construction of onemodified iris;

Fig. 4(a) is a detailed sectional 'view showing a construction ofasecondmodified iris;

Fig. 5 is an isometric view of a modified form of cavity resonatorparticularly suitable for use in connection with a microwaverefractometer;

Fig. 6 illustrates an assembly technique employed in producing thestructure ofyFig. 5, and V t Fig. 7 shows the modified cavity resonatorof Fig. 5 assembled in a mounting.

In general in accordance with the principles of the presentinvention-the cavity resonator is fabricated from ceramic material byfirst forming a particular selected ceramic into a. generallycylindrical or other hollow shape and curing it by well-known refractorymethods to form a moulding approximating the dimensions of the desiredcavity resonator. For convenience, one end wall of said cylinder ormoulding maybe formed integrally with the cylinder, a closure memberbeing provided for the other end wall to permitmechanical op- .erationson the internal surfaces of the cavity. Alterspecific manner in whichsuch. outlined procedure is implemented will be further detailed in theparticular examples enumerated below.

Fig. 1 shows a completed microwave cavity resonator constructed inaccordancewith the principles of the present invention. Fig. 1 shows thecavity with the cover plate removed to provide a view of the interior ofthe. cavity and withone of the. waveguides detached in order. to showthe construction of one form of the,

' iris.

with a cylindrical cavity 2 which maybe finish-machined .orotherwiseformed to exacting dimensions.

Portions of the outside of the body are suitably flattened to providebosses- 3 which are adapted. to receiveithe flanges' 4 of thewaveguidesw 5; The region defined by the bosses 3 are further providedwith suitable microwave irises 6 has a verylow'temperature coeflicientof frequency variafor, use in connection with a microwavefrefractometerfor measuring'refractive indexes of gases,

Still another ;object of the present invention is to providea cavityresonator. which is highly stable and rela tively insensitive totemperature effects;

cavity resonator of. high thermal stability which can; be

produced easilyand economically;

A San further object of this invention isf/to sm a which may be formedeither bybcring through. thewall.

.' of a cavity or which maybe leftsolid in accordance with theprinciples subsequently to be described.

The interior-surface of the cavity'Z is provided with ,a-

surface of electrically conducting material which may be brushed,sprayed, electrodeposited, or :otherwise'applied-. to the interiorsurface of the cavity. The waveguides 5 are conventional and require nofurtherdescr'iption with the" exception thatthe flange 4 thereof ispreferably made of Invar or other material having a lowtemperature co- 1eflicient'of'expansion. {Suitable receseses 3a.for fastening elementsare provided in the y of the cavii land. these recesses. are providedwith inserts 7 for, th'readingly s A receiving suitable fasteners aswill be described; [The in- Other uses and; advantages of theiinventioni will be- "come. apparent upon reference to thefspecification and drawings, in which I Fig. '1 is" a disassembledViewQQf ne ofcavity.

sert's as well-as. the fasteners are made of temperature- I:

insensitive material "as will-be described l to inhibit lithe effect [oftemperati res ion the geometrical size and, con j sequently; thefrequency of the cavity; Alternat'ely asiwill 1 be described, the coverfnember8 whichis showndetached The interior ing a closure membertherefor.

isatis factory. ceramic surfaces to be joined-with a thickcoat of theabove-identified conducting paint. 'The surfaces to be r 3 from thecavity in Fig. 1 may be cemented or soldered to the cavity without theuse of fastenings to further reduce the possibility of temperatureeffects consequent to ,the use of metallic fastenings.

gether with its closure member 8. Such cavity as will be apparent canreadily be constructed by conventional moulding techniques whicharecommonly employed in the ceramic art. Specifically, materials such asceramics identified as McDanelL-53A and 581-G made by the McDanelRefractory Porcelain Company of Beaver Falls,

Pennsylvania, may be employed. Such ceramic material is formed in theshape of :a right circular cylindrical cup as shown in Fig. 3 togetherwith a flat circular disc form- The cups are initially fabricated bymoulding techniques to provide a cavity having a geometry conforming tothe particular size of resonant cavity desired.

The ceramic material after moulding and curing is easily and readilyworked by conventional mechanical means such as grinding and machiningto provide the necessary precise final dimensions. Tolerances of athousandth of an inch are readily obtainable, care being exercised tohold the ceramic materials during the process types of conducting paintare available for such purpose.

Specifically, Du Pont Type F? silver paste No. 6449 and 6296 areparticularly suitable. Such pastes are thinned to brushing consistencyusing butyl Cellosolve and the paste is then applied to the criticalsurfaces of the cavity by brushing, spraying, or dipping and thenairdried for about an hour. Subsequently, the cavity is fired in anelectric furnace through cycles recommended by the manufacturer of theconducting paste. For Du Pont No. 6296 the procedure entails raising thefurnace within which the ceramic has been placed to a temperature fromambient to about 1385 F., soaking the ceramic at such temperature forabout} or 4 minutes then shutting off the furnace and allowing it tocool slowly to room temperature before removing the ceramic. Whenapplying No. 6449 conducting paint the furnace is fired in the samemanner except that a peak temperature of only about 1050 to 1100" F. isemployed. In either instance, the first and second coats of theconducting paste are absorbed to a considerable degree by the relativelyporous ceramic and usually three coats are provided on the cavity in theabove-described manner in order to obtain a layer of adequate thicknessand uniformity.

The ceramic cup and cover" after being treated with conducting materialin the above-described manner, are I then machined in order to providefastenings for attach- .ing the waveguides 5 and end plate 8. Asindicated in Fig. 1 suitable holes 3a are drilled in the ceramic bodywhere necessary to provide fastenings for the cover {plate 8 andtheflanges 4 of the waveguides. I Fitted inserts 7 made from'commercialNilvar an Invar alloy made byth e Driver-Harris Corporation are thencemented into each of these holes. .the inserts in the holes comprises apaste made from pul- The material forcementing verized McDanel L-53Aceramic combined with sodium silicates In addition, another method offastening was found Such method consisted .of coating the joined such asthe cover plate S were then clamped firm- 4 above-described manner. Suchtreatment resulted in a firm bonding.

As indicated in Figs. 1 and 2, irises for the transmission of microwaveenergy may be provided in the body or wall portion of the cavity byboring suitable recesses 6a and holes 6 through the cavity wall.

Fig. 4 shows an alternate embodiment singularly adapted to the presentinvention for forming microwave irises without the necessity of boringan opening through the wall of the ceramic body. As indicated in Fig. 4a portion 6b of the electrically-conductive surface corresponding to thesize and location of the iris is removed either by scraping or byinitially masking such area so that the electrically-conductive surfaceapplied in the previously-described manner will not adhere to suchportion (6b) of the wall of the ceramic body. Such opening provides anexcellent microwave conducting port despite the fact that the body ofthe ceramic material is not removed. It was found that a wall thicknessof ap proximately 0.030" was sufficient. Such wall thickness not onlypreserves the strength of the cavity but readily permits thetransmission of microwave energy. In effect, such method providesamicrowave coupling iris which literally is filled with ceramic insteadof air.

Alternately as shown in Fig. 1 the irises are made merely by boringsmall holes 6 in the end walls of the cavity and lining the holes withthe conducting material in the above-described manner.

Specific examples of cavities made in accordance with the above-outlinedprocedures will follow.

Example 1.A cylindrical ceramic body 1 together with a cover plate 8 asshown in Fig. 3 were fabricated by usual ceramic moulding techniquesusing the above-described ceramic material corresponding to McDanel L-53A and 581-G as above identified. The material was first formed into acylindrical cup 1 (Fig. 3) having a closed bottom and an open toptogether with a disk 8 approximately the diameter of the cup to serve asa cover. The interior cylindrical surface of the cup was carefullymachined and ground as by centerless grinding techniques to provide acavity of desired dimension commensurate with the desired resonantfrequency of the cavity. The bottom of the cup was also finished aswasthe fiat face of the cover member.

Diametrically opposed portions of the outside cylindrical portion of thecup were then flattened to provide the bosses 3 described in connectionwith Fig. 1 by grinding. A portion 6a on each of such bosses 3 was thenfurther reduced by counter. boring to provide a ceramic wall thicknessof approximately 0.030". Such counterbores were provided on each boss 3in alignment with each other. A small iris hole was then drilled throughthe Wall of the cup within the counterbore 6a to provide a microwaveiris. The size of holes employed ranged from /8" to A" in diameter. Theentire surface of the cup together with the cover plate was thencarefully coated with Du Pont silver paint as above identified. v

The bore of each iris was also coated with such conducting paint. Boththe cup and the cover plate were then placed in an electric oven andtreated in the manner described above and such treatment was thenfollowed by the application of suflicient number of additional coats ofsilver paint until a uniform conducting surface was obtained on the cupand cover plate. Suitable holes were then drilled in both thecover plate8 and in the rim of the cup as shown in Fig. 1 corresponding to theposition of each of the inserts 7. Threaded the waveguide. T he holes inthe bosses were then filled r with Invar inserts in the same manner.Such cavity was found to be resonant at 9319 me. and had a Q of 11,740,a 1 I Example 2.-A cavity-was constructed in accordance with theprocedure detailedin Example 1 except that the '7 cavity was made fromthe ceramic corresponding to McDanel 581-G ceramic. The cavity wasprovided with an inside diameter of about 1.72 and an interior length orheight of about 1.36" such cavity upon test was found to have a resonantfrequency of 9,304 me. and a Q of about 13,650. 7

Example 3.-A cavity was fabricated in accordance with the steps outlinedin connection with Examples 1 and 2 except no through holes were boredto provide irisesr Specifically, theportion of the wall of the ceramicat the bottom of each of the counter-bores 6a were carefully maskedduring the coating operation to provide an area 6b (Fig. 4)approximately corresponding to that of the bored irises. The resultingconstruction was a ceramic cup having solidwalls in which the iriseswere ceramic filled instead of being open. Such cavityhaving solidirises was found to have a resonant frequency of response of 9,306 me.and a Q of 4400. Various sizes of such unpainted holes forthe iriseswere tried ranging down to diameter on the inside and Mt" diameter onthe outside. The Q did ,notappreciably change but the transmission losswas found to be higher than that of the open type of iris.

Example 3A.-A still further modification of the iris constructionsimilar to Example 3' was made by boring a hole through the wall of theceramic body such as the hole 6a, Fig. l, and lining the hole withconductive material as described in connection with Example 1 The holewas then filled with ceramic paste (Fig. 4A) made of pulverized L-53Aceramic and sodium silicate. The ceramic body was then provided with aconducting surface, the iris being formed by-rnasking as described inconnection with Example 3. The transmission loss was. substantiallyreduced from that of Example 3.

In all of the above examples, the end plate 8 was secured to the cup ofbody 1 by means of machine screws made of Nilvar which passed throughthe holes in .the

plate 8 into the Nilvar threaded sleeves orinserts 7 provided in thebody portion of the cavity 1. The wave- 6 serted in the slots and thering 8 is then mounted in the nest formed by the inner ends of thestruts. The

. struts are soldere into the slots in the cavity wall and the ring tothe strut ends with the Du Pont silver paint used to coat the. cavitysurfaces, applied. rather heavily and fired. according to the cycle.previously described. If desired, the cavity may, optimally be providedwith a protective jacket such as the metallic casing 11 shown in Fig. 7.

Such described embodiment illustrates the ease with which complex shapedcavities can be fabricated in accordance withthe principles of thepresent invention.

It is apparent from the above description that in accordance with theprinciples of the present invention resonant cavities having a highdegree of temperature stability can readily be constructed cheaply andin large quantities. While the ceramic bodies 1 in connection .with thepresent case were moulded by conventional invention it will be readilyapparent that many varieties ceramic moulding techniques, followed bycareful machining of the interior surfaces after curing, it is obviousthat by use of precision dies, cavities having accurate dimensions toproduce the desired degree of resonance can be directly moulded followedby very little or no machining operations.

Moreover, as above noted, fastening of the cover plate and waveguide tothe body 1 is readily accomplished by using the conducting surfacingpaste as a cement, or alternately, by soldering the conducting surfacestogether. Also other cements may beemployed for bonding. The use of theInvar fastenings is thereby dispensed with.

a It will be apparent thatin accordance with the principles of thepresent invention the need-for expensive temperature-insensitivematerials is either minimized or disinsensitive alloys is materiallyreduced.

While particular ceramic materials and electrically conducting coatingshave been specificaly identified as examples to enable construction andpractice of the present of commercially available ceramic materials andelecthe dimension of the flat. 3 formed in the body of the cup, theflange '4 of the transmission line corresponded generally to the outsidedimensions of thecup; The

flange of the Waveguide wassecured to the body 1 by means of machinescrews fitting into Nilvar threaded use in' microwaverefractometersfwhere the test gas of continuously-varying r'efractivity.may; be introduced into I the test cavity rapidly and withlittle flowresistance, and its eifect upon the resonant frequency of the "cavitybeing recorded, with'rninimum error due to temperature vari?'ationsaltering the size or shapelof the cavity.

trically conducting coatings can readily be employed in accordance withthe principles of the present invention. It will beapparent that theembodimentsshown are only exemplary and that various modifications canbe made in construction and arrangement within the scope of invention asdefined in theappended claims.

What is claimed is: l. A resonator having a desired resonant frequencycomprising a body of material having a low coefficient of expansion andcontaining a hollow volume bounded by at least one surface on allexcepta first and second side, .the volume having a configuration and sizedependent upon said resonant frequency and said first and second sideallowing the free passage of a gas through said volume, a coating ofelectrically conductive material bonded to said surface, a-first andsecond hollow ring, means Specifically, the-,bodyla of thec'avity ismaster lowing the procedure outlined in connection with Examples 1 and2. The front aud back'por'tions of the cavity are leftopen'.Sui-tablebosses 3b are milled iirxthe body: to provide mounting surfacesfor the waveguides 5.

" Orific'es in the form of cylindrical rings are then formed of ceramicmaterial ari d theyare then 'inserted for mounting said, first andsecond ring in said first and second side, respectively, said lastmentioned means permitting-the passage of a gas through said first andsecond side,- and means for coupling electromagnetic energy through saidsurface and through said volume. a 2.. resonator having a desiredresonant frequency comprising .a bodyof 'material having a lowcoefiicient ,of expansion and containing ahollow volume bounded .by atleast one surface on all excepta first and second side, theyolume havinga configuration and sized deconcentricallywiththe cylindricalbody In bymeans of struts 9"which are also made of ceramic strips.

Figiy6 sho'ws onemethod bfassembling .th'ej cavity 7 shown in..Figs-.5fand 17 Slots 10 maybe-,milledjinlthe" periphery of the cavity 1a. QThestruts' 9 arethen inpendent upon said resonant frequency and said firstand second side allowing thefreepassage of a gas through said-volume, acoating ofelect'rically conductive material ,bonded tosaid surface, afirst and second hollowring, at

.least a firststrut mounting said first ring in said first side,

atjl'east a second strut mountingsaid second ring in said 7 second side,and means for coupling electromagnetic energy through said surface andthrough said volume.

References Cited in the file of this patent UNITED STATES PATENTS2,281,247 Peterson Apr. 28, 1942 8 White Sept. 26, 1950 Rosencrans' Mar.22, 1955 Foster June 10, 1958 McArthur Nov. 11, 1958 Ashbaugh July 28,1959

